The present invention relates to navigation and, more particularly, to a method for vehicular navigation utilizing signals transmitted from two or more known positions.
Navigation systems for vehicles such as ships and airplanes have evolved through various stages of development, commencing of course, with simple celestial systems. One of the first all-weather electronic navigation systems was the LORAN (long range navigation) system. The LORAN system operates on the assumption that two pulses of wave energy synchronously transmitted from widely separated transmitters will arrive simultaneously at any location equidistant from the transmitters. Thus a line of equidistance between the transmitters defines positions at which the signals from the transmitters will be received simultaneously. At all other positions relative to the transmitters, one signal will arrive sooner than the other and curved lines representing positions of equal time difference can be drawn. The result of plotting this information is a family of hyperbolic curves that represent lines of position. If a second pair of transmitters is located so that its lines of position cross those of the first pair of transmitters, a complete position locating system exists.
Essentially, a typical LORAN system operates in this manner and is therefore typically referred to as a hyperbolic system. It should be noted however, that in practice the LORAN system typically employs three transmitters arranged in a triangular configuration with one station being common to both pairs. Moreover, the signals are typically not transmitted simultaneously but at precisely spaced intervals.
One drawback of LORAN systems is decreased accuracy at long ranges. At relatively close ranges where the hyperbolic lines cross at close to right angles, any inaccuracies caused by system errors are minimized since possible position error due to jitter of the crossing lines is circular and position is known to be within that relatively small circle. However, when the lines approach parallelism at the longer ranges, any position error produces a long narrow ellipse and since actual position may be anywhere within this ellipse, position error is considerably greater as range increases.
One improvement in the LORAN system has been to use direct ranging techniques so that the lines of position are circular rather than hyperbolic. Direct ranging techniques involve the computation of range from two or more known positions to the vehicle so that at least two intersecting circles of position may be used to locate the position of the vehicle.
One such direct ranging system operates in conjunction with a conventional LORAN receiver to provide ranges to two or more LORAN stations so that positions may be determined. With this system, the navigation computer assumes a transmission time for the LORAN signals and then makes calculations based upon the actual arrival times of the signal to see if the assumption was correct. If the assumption was correct, the three ranges from the LORAN stations will form circles which cross at one point. If the circles do not cross at one point, the system makes a correction based upon the first answer and assumes another transmission time. By this method, the computer can converge on the exact range from each station and can thus determine the position of the vehicle.
While the direct ranging approach to navigation is more accurate than the hyperbolic approach of conventional LORAN systems, difficulties are still encountered because of propagation characteristics of the transmitted wave energy over different types of terrain and because of system timing inaccuracies and the like.
The present invention relates broadly to "radio navigation" as distinguished from the other conventional categories of navigation, i.e., pilotage, celestial navigation, and dead-reckoning. More particularly, the present invention relates to those radio navigation techniques/equipment/instrumentalities which provide position information based on the utilization of the finite, nearly constant, rate of electromagnetic wave propagation, i.e., on appropriate time or phase measurements. Examples of conventional radio-navigation systems include the "hyperbolic" type employing signal time-of-arrival-differences such as Loran, and hyperbolic types employing signal phase-of-arrival-differences such as Omega, and Decca. The present invention relates to "Direct Ranging" modes of some existing equipment, e.g., Loran, and also comprises a direct ranging (related) mode for signal phase-of-arrival-difference-measuring equipment such as conventional systems as Omega and Decca.
Contemporary systems-engineering practice calls for the use of any one, or any combination, of the four types of navigation systems (pilotage, celestial, dead-reckoning, radio) in a given vehicular system. In a given application, the systems-engineering practice entails judicious selection from all available types and techniques in order to achieve the desired level of accuracy, reliability, range, "security," automatic operation, ease-of-use, economy, size, weight, environment, servicability, etc. The present invention is compatible with the aforesaid "systems" approach to navigation, in that it embodies advances in radio-navigation apparatus.
It is accordingly an object of the present invention to provide a novel method of radio navigation having advantages over related radio-navigation methods.
It is a further object of the present invention to provide a novel method of radio navigation which is compatible with existing radio navigation system hardware.
It is another object of the present invention to provide a novel radio navigation method in which signal perturbations have less effect on position accuracy than in known systems.
It is yet another object of the present invention to provide a vehicular navigation method utilizing time difference information with respect to successive transmissions from each of two or more transmitter sites to provide radial increment of distance information for dead (deduced) reckoning type of position calculations.
These and other objects and advantages are accomplished in accordance with the present invention as will become apparent to one skilled in the art to which the invention pertains from a perusal of the following detailed description when read in conjunction with the appended drawings.